The Hammer is a sticker found in Paper Mario Sticker Star. It can damage multiple foes via a shock wave produced when using the hammer. If you wait too long before using it however, the top will fall off dealing a very weak amount of damage.
In-game Descriptions[]
Regular: Strikes foes with a swung hammer. Can damage multiple foes but won't hit flying ones.
Museum: A hammer that can pound enemies flat but is useless against flying enemies. Use it wisely.